Trends in land-use change related to algal biofuel production are important to quantify. However, until there is a history of commercial development of algal biofuel production facilities, probable land-use changes and trends will need to be projected based on economic and social drivers and environmental contributing factors.
Where important or rare ecosystem services are provided by the baseline land use, a measure of those services could serve as a sustainability indicator for algal biofuels. The services of pastures, rangelands, and coastal waters that might be displaced by feedstock production facilities would be important to quantify. Relevant metrics would be:
• National or regional area of grassland and shrubland devoted to livestock grazing; however, data are lacking on the acreage used for livestock grazing (The H. John Heinz III Center for Science and the Environment, 2008).
• Number of livestock fed on grasslands and shrublands (West, 2003; The H. John Heinz III Center for Science and the Environment, 2008).
• Pasture yield calculated on a per-area or per-forage biomass basis (methods described in Burns, 2008).
A less direct indicator of livestock numbers or biomass would be area covered by grassland and shrubland (West, 2003; The H. John Heinz III Center for Science and the Environment, 2008). Additional sustainability indicators have been suggested for brownfield redevelopment efforts. Some of these are summarized in Wedding and Crawford-Brown (2007) and would be appropriate where algal biofuel production is sited on brownfields.
The potential to mitigate GHG emissions is one of the motivations to develop biofuels. The basis of mitigation is that carbon emissions from combusting a biofuel are cancelled by the corresponding capture in photosynthesis. This said, the net GHG emissions of producing biofuels and coproducts are not zero because of carbon and other GHGs emitted in processing. In this section, the results of life-cycle assessment (LCA) studies of GHG emissions are reviewed critically.
5.3.1 Life-Cycle GHG Emissions of Algal Biofuels
Primary GHG emissions from algal biofuels are expected to be connected to the use of energy in the processing chain (see section Energy in Chapter 4). The translation of energy use to GHG emissions is complicated by variability in the carbon overhead of different forms of energy, in particular electricity. The average direct GHG emissions of electricity production in the United States is 606 grams of CO2 equivalent per kilowatt hour (EIA, 2002). Depending on the mix of fossil fuels, hydropower, nuclear, wind, and other sources providing power to the grid, emissions vary by state from 13 to 1,017 grams CO2 equivalent per kilowatt hour (EIA, 2002). The approach taken by many analysts is to use a national average emission factor (Liu et al., 2011).
LCA results for net GHG emissions for algae biofuel production vary from a net negative value (that is, a carbon sink) to positive values substantially higher than petroleum gasoline (Table 5-4). As with the case for energy use (see Chapter 4), drivers of variability in CO2 emissions are nutrient source, productivity and process performance, and the credit associated with coproducts. For example, Sander and Murthy (2010) assumed that residual algal biomass substitutes for corn in ethanol plants. Corn is energy intensive to produce; the